With the advance of techniques for manufacturing thin-film transistors, the liquid crystal displays (LCD) are widely applied in electrical products, such as PDAs, laptops, digital cameras, video cameras, cellphones, high resolution television sets, etc. due to advantages as smaller size, portability, non-radiation and lower power consumption. Especially when vast amounts of resources have been expended in researching and developing improved materials, processes and equipments to manufacture LCD devices, the qualities of the LCDs are promoted and costs thereof are reduced substantially. Notedly, it is required to introduce backlight units into the LCDs for illumination because the liquid crystal molecules are non-illumination materials. Therefore the backlight unit is the most importance module for manufacturing the LCD devices, and the performance thereof is closely related to the displaying effect of the LCD.
Refer to FIG. 1, the backlight unit 1 applied to the current LCDs comprises an optical film 10, a diffuser plate 12, a reflective plate 14 and a plurality of lamp tubes 16. These lamp tubes 16 are disposed under the diffuser plate 12 with the selected interval. The light of the lamp tubes 16 illuminating into the diffuser plate 12 can be diffused more uniform and then emit upwardly through the upper surface of the diffuser plate 12.
For promoting the uniformity and brightness of the light in advance, on the diffuser plate 12 usually disposed is the aforementioned optical film 10. In general, the optical film 10, according to different functions, includes an upper diffuser film, a brightness enhanced film (BEF), and a lower diffuser film. The lower diffuser film is heavy misted to spread the light uniformly and to make the sharpness more smooth, while the brightness enhanced film is made of prizm or polarizer to aggregate light. As to the lightly misted upper diffuser film is applied to spread light uniformly and protect the films beneath from scrapes in later package procedures.
Besides, for preventing the light of the lamp tubes 16 from escaping downwardly, the reflector plate 14 is disposed under the lamp tubes 16 to reflect the light emitted downwardly, thereby promoting the light use efficiency. Namely, because the lamp tubes 16 are disposed in the space enclosed by the diffuser plate 12 and reflector plate 14, the light emitted out of lamp tubes can be accumulated onto the diffuser plate 12 entirely, so as to promote the illumination efficiency of the backlight unit 1.
It is noted that the current direct type backlight unit 1 has some drawbacks. Please refer to FIG. 1, the light emitted from the lamp tubes 16 are delivered along two directions mainly, the first direction is from the lamp tube 16 direct to the diffuser plate 12, and the second direction is from the lamp tube 16 to the reflector plate 14 and then reflecting back to the diffuser plate 12. Because the brightness is in inverse ratio to the distance, for the first direction, the lower surface of the diffuser plate 12 right above the lamp tube 16 will have the more bright light, while the other portions of the lower surface above the left and right sides of the lamp tube 16 will have the less bright light. As for the second direction, because the most reflected light from the reflector plate 14 is centralized on the surface of the diffuser plate 12 right above the lamp tube 16, the light brightness of the lower surface of the diffuser plate 12 will have light and shade contrast distinctly, thereby lowering the light uniform of the direct backlight unit 1.
Please refer to FIG. 2, the brightness diagram of the current direct type backlight unit 1 is measured. It is clearly illustrated the light brightness has the acute light and shade contrast with the distance. For example, the diffuser plate right above the lamp tube 16 has the more bright light of 9000 ANSI, while the diffuser plate between two adjacent lamp tubes 16 has the less bright light of 4000 ANSI. Under such condition, even introducing the diffuser plate 12 and the optical film 10, the whole illumination of the direct type backlight unit 1 is still not uniformly, thereby causing the screen appearance of the display have light and shade distinction.
For overcoming the issue of light spreading uniformly, in conventional techniques, a screen printing procedure is introduced to fabricate scattering dots on the diffuser plate 12 for enhancing light diffusing effect. However, it is required to perform an extra printing procedure on the diffuser plate 12, and also increase the production costs of the backlight unit 1. Besides, another solution, in the conventional techniques, is to increase the distance between the diffuser plate 12 and the lamp tube 16 for well uniforming the light emitted out of the lamp tube 16 to the diffuser plate 12. But this solution will cause the thickness of the backlight unit 1 increase so that the backlight unit manufactured can not satisfy the portable tide.